Thermal printer and resistance data measuring device for thermal head of the same

ABSTRACT

A thermal head of a thermal printer has plural heating elements arranged along a line and connected in parallel with one another. A respective one of the heating elements is connected to one of plural heat control switches, which are selectively turned on/off for driving the heating elements individually by applying electrical energy to generate heat. To measure resistance data of at least one of the heating elements, a capacitor is connected in parallel with the plural heating elements. A reference resistor is connected in parallel with the plural heating elements and the capacitor. The capacitor is charged, and then discharged via the reference resistor or the one of the heating elements. A voltage across the capacitor is detected. An amount of discharging time required to decrease of the capacitor voltage from a predetermined high voltage to a predetermined low voltage is measured while the capacitor is discharged, in association respectively with the reference resistor and the heating elements. Resistance data for the heating elements is determined in accordance with the discharging time of the heating elements, respectively, with reference to the discharging time of the reference resistor.

This application is a divisional of co-pending application Ser. No09/175,573, filed on Oct. 20, 1998, which is a divisional of applicationSer. No. 08/749,546, filed on Nov. 15, 1996, U.S. Pat. No. 5,852,369 theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal printer and a resistance datameasuring device for a thermal head of the same. More particularly, thepresent invention relates to a thermal printer in which irregularity inperformance of heating elements of a thermal head can be overcome in aprinting operation, and a resistance data measuring device for a thermalhead of the thermal printer.

2. Description Related to the Prior Art

Conventional thermal printers include a thermal transfer printer inwhich ink film is used, and a direct thermal printer in whichthermosensitive recording material is heated for directly printing animage.

The color thermal printer of the direct printing uses a colorthermosensitive recording material or recording sheet, in whichthermosensitive coloring layers of magenta, yellow and cyan are formedon a support. To develop colors on, the coloring layers applied to thecoloring layers the amount of heat energy (in mJ/mm²) to is varied. Thelowest heat energy is required for coloring one of the coloring layerslocated at the obverse of the recording sheet. Higher heat energy isrequired for coloring the coloring layers according to the closeness tothe support. Once a coloring layer is heated, electromagnetic rays areapplied to it to fix that layer before heat energy is applied forcoloring the next layer to be colored. This process is performed toinhibit further coloring by the present coloring layer effectivelypreventing that layer from being colored beyond a desired density.

The thermal head includes an array of heating elements as resistors,which are arranged to record pixels arranged in one line. To record animage of each color thermally, heat energy is applied to the recordingsheet as a sum of bias heat energy and image heat energy. The bias heatenergy has an amount slightly short of causing the coloring layer todevelop the one color, and is applied to the recording sheet during thebias heating at the beginning of recording each one pixel. The imageheat energy has an amount determined according to the gradation level ofone color, namely coloring density of the pixel to be printed, and isapplied to the recording sheet during the image heating which succeedsthe bias heating.

To reproduce high gradation, the heating operation is controlled finely.The heating elements of the thermal head need to have an equalresistance for the purpose of precise application of the heat ascontrolled. It is however inevitable that the heating elements haveirregularity of 5-10% in resistance. If the heating elements are drivenfor an equal duration, generated heat energy differs between the heatingelements due to the differences in the resistance. Irregularity indensity is likely to occur in an image being recorded.

U.S. Pat. No. 5,469,068 (corresponding to JP-A 6-79897) discloses athermal printer in which the resistance of the heating elements ismeasured for the purpose of preventing occurrence of irregularity inprinted density by compensating image data. The thermal printer isprovided with a capacitor, having a known capacitance. The capacitor ischarged fully, and then discharged via the heating elements connectedthereto. The time for a capacitor voltage to decrease is measured. Forexample, the decrease of the capacitor voltage down to a half of a powersource voltage is checked to measure the discharging time. According tothe discharging time and the capacitance of the capacitor, theresistance of the heating elements is calculated in view of aproportional relationship between the discharging time and theresistance of the heating elements.

This prior document also suggests use of a reference resistor to whichthe capacitor is connected, and of which resistance is known. Thecapacitor is charged fully, and then discharged via the referenceresistor. The discharging time for decrease in the capacitor voltage ismeasured until a predetermined voltage is reached. Again the capacitoris charged fully and discharged via the heating elements. Thedischarging time for decreasing the capacitor voltage is measured untilthe predetermined voltage is reached. According to the resistance of thereference resistor and the discharging times via the reference resistorand the heating elements, the resistance of the heating elements iscalculated.

According to U.S. Pat. No. 5,469,068 (corresponding to JP-A 6-79897),the capacitor is fully charged by applying voltage for a predeterminedduration, and then discharged down from the capacitor voltage equal tothe power source voltage, until the discharging time is measured. Tomeasure the resistance of the heating elements with precision, theduration for the charging operation should be long enough ensuring theensure full charging. A problem of the prior art lies in considerableslowness of measuring the resistance of all the heating elements. If theduration for the charging operation is shortened, the capacitor voltageupon the finish of the charging is not kept equal due to the chargehaving initially remained in the capacitor. Another problem lies in lowprecision in the measurement of the resistance.

SUMMARY OF THE INVENTION

In view of the foregoing problems, an object of the present invention isto provide a thermal printer in which irregularity in performance ofheating elements of a thermal head can be overcome in a printingoperation, and a resistance data measuring device for a thermal head ofthe thermal printer.

Another object of the present invention is to provide a thermal printerin which resistance of heating elements can be measured precisely andquickly, and a resistance data measure device for a thermal head of thethermal printer.

In order to achieve the above and other objects and advantages of thisinvention, a thermal head has plural heating elements arranged along aline and connected in parallel with one another. A respective one of theheating elements is connected to one of plural heat control switches.The heat control switches are selectively turned on/off for driving theheating elements individually by applying electrical energy to generateheat. A capacitor is connected in parallel with the plural heatingelements. A charging switch is adapted to charging the capacitor. Avoltage detector detects a capacitor voltage across the capacitor. Areference resistor is connected in parallel with the plural heatingelements and the capacitor. An additional switch is connected to thereference resistor. A control circuit controls the heat controlswitches, the charging switch and the additional switch. The controlcircuit initially turns on the charging switch to charge the capacitor.The charging switch is turned off to stop charging the capacitor upon anincrease of the capacitor voltage to a predetermined high voltage. Oneselected from a group including the additional switch and the pluralheat control switches is thereafter turned on, to discharge thecapacitor via the reference resistor or one of the heating elements inassociation with the selected one being turned on. A timer measuresdischarging time elapsed in a decrease of the capacitor voltage from thepredetermined high voltage to a predetermined low voltage while thecapacitor is discharged, in association respectively with the referenceresistor and the heating elements. A resistance data determinerdetermines resistance data of the heating elements in accordance withthe discharging time respectively of the heating elements with referenceto the discharging time of the reference resistor.

In a preferred embodiment, the resistance data is a ratio of thedischarging time of each of the heating elements to the discharging timeof the reference resistor, and represents a relative greatness ofresistance.

The resistance data measure device is incorporated in a thermal printerin which the heating elements are respectively driven by a drive signalbased on bias data and image data, to effect thermal recording torecording material. The thermal printer further includes a compensatorfor compensating the drive signal associated with the heating elements,in accordance with the resistance data determined by the resistance datadeterminer.

In a variant, a reference resistor is connected to the charging switchin series. A control circuit controls the heat control switches and thecharging switch. The control circuit initially turns on the chargingswitch to charge the capacitor via the reference resistor, turns off thecharging switch to stop charging the capacitor, and thereafter turns onone selected from the plural heat control switches, to discharge thecapacitor via one of the heating elements in association with theselected one being turned on. A timer measures charging time elapsed inan increase of the capacitor voltage from a predetermined low voltage toa predetermined high voltage while the capacitor is charged. The timermeasures discharging time elapsed in a decrease of the capacitor voltagefrom the predetermined high voltage to the predetermined low voltagewhile the capacitor is discharged, in association respectively with theheating elements. A resistance data determiner determines resistancedata of the heating elements in accordance with the discharging timewith reference to the charging time.

It is preferable in the variant that the control circuit turns off thecharging switch upon an increase of the capacitor voltage to thepredetermined high voltage while the charging switch is turned on.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore apparent from the following detailed description when read inconnection with the accompanying drawings, in which:

FIG. 1 is an explanatory view in elevation, illustrating a mechanicalconstruction of a color thermal printer;

FIG. 2 is a block diagram schematically illustrating a resistance datameasuring device and circuits relevant to a thermal head;

FIG. 3 is an explanatory sectional view, illustrating layers of a colorthermosensitive recording sheet;

FIG. 4 is a graph illustrating coloring characteristics of the recordingsheet;

FIG. 5 is a block diagram schematically illustrating an electricalconstruction of the thermal printer;

FIG. 6 is a flow chart illustrating determination of resistance data;

FIG. 7 is a flow chart illustrating measurement of discharging time viathe reference resistor;

FIG. 8 is a flow chart illustrating measurement of discharging time viaeach heating element;

FIG. 9 is a timing chart illustrating signal waveforms duringmeasurement of the discharging times;

FIG. 10 is a block diagram schematically illustrating another preferredresistance data measuring device in which charging time and dischargingtime are measured;

FIG. 11 is a flow chart illustrating determination of resistance dataaccording to the resistance data measuring device of FIG. 10; and

FIG. 12 is a timing chart illustrating signal waveforms duringmeasurement of the charging and discharging times.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENTINVENTION

In FIG. 1, a platen drum 10 is mounted about a rotational shaft 11, androtated by a stepping motor 12 in a sub-scanning direction indicated bythe arrow. A color thermo-sensitive recording sheet 13 is mounted on aperiphery of the platen drum 10. A front edge of the recording sheet 13is fixedly retained on the platen drum 10 by a damper 14. The damper 14is movable between a position on the platen drum 10 for the retention ofthe recording sheet 13 and a position away from the platen drum 10.

Near to the periphery of the platen drum 10, there are disposed athermal head 16, a yellow fixing optical device 17, and a magenta fixingoptical device 18. In FIG. 2, the thermal head 16 includes a heatingelement array 20 in which heating elements 20 ₁-20 _(n) are arrangedalong one line in a main scanning direction, namely in parallel with theaxis of the platen drum 10. At the time of printing, the heating elementarray 20 is pressed against the recording sheet 13. The yellow fixer 17includes an ultraviolet lamp 17 a and a lamp housing 17 b. The lamp 17 aemanates ultraviolet rays peaking at the wavelength of 420 nm. Themagenta fixer 18 includes an ultraviolet lamp 18 a and a lamp housing 18b. The lamp 18 a emanates ultraviolet rays peaking at the wavelength of365 nm.

In FIG. 3 illustrates a layered structure of the recording sheet 13. Therecording sheet 13 includes a support 23, a cyan coloring layer 24, amagenta coloring layer 25, a yellow coloring layer 26 and a protectivelayer 27 disposed in the order listed. The magenta coloring layer 25 hasoptical fixability responsive to ultraviolet rays of a wavelength rangeof nearly 365 nm. The yellow coloring layer 26 has optical fixabilityresponsive to ultraviolet rays of a wavelength range of nearly 420 nm.The recording operation is effected in the order from the obverse towardthe support 23, namely in the order of yellow, magenta and cyansuccessively. It is also possible to use an alternative recording sheetincluding the support 23, the cyan coloring layer 24, the yellowcoloring layer 26, and the magenta coloring layer 25 disposed in theorder listed. With this recording sheet, recording operation is effectedin the order magenta, yellow and cyan. Note that in FIG. 3, Y representsthe yellow coloring layer 26, M represents the magenta coloring layer25, and C represents the cyan coloring layer 24.

There are intermediate layers which are disposed between the coloringlayers 24-26 for regulating thermal sensitivity of the coloring layers24-26, but which are not shown in the drawing. The support 23 consist ofa piece of opaque coated paper or plastic film. Also transparent plasticfilm may be used for producing an OHP sheet adapted to an overheadprojector (OHP).

FIG. 4 illustrates coloring characteristics of the coloring layers24-26. Higher heat energy is required for coloring the coloring layers24-26 according to the closeness to the support 23. The lowest heatenergy is required for the yellow coloring layer 26. The highest heatenergy is required for the cyan coloring layer 24. To record a yellowpixel thermally, heat energy is applied to the recording sheet 13 as asum of bias heat energy Eby for yellow and image heat energy Egy foryellow.

The bias heat energy Eby has an amount slightly short of causing theyellow coloring layer 26 to develop the yellow color, and is applied tothe recording sheet 13 during the bias heating at the beginning ofrecording each one pixel. The image heat energy Egy has an amountdetermined according to the gradation level of yellow, namely yellowcoloring density of a pixel to be printed, and is applied to therecording sheet 13 during the image heating which succeeds the biasheating. Similarly the bias heating and the image heating are effectedby applying magenta bias heat energy Ebm, magenta image heat energy Egm,cyan bias heat energy Ebc and cyan image heat energy Egc.

FIG. 5 schematically illustrates the color thermal printer. A systemcontroller 30 effects preset sequences, and controls a section for thethermal recording, a resistance data measuring device 50 (see FIG. 2),the stepping motor 12, the yellow fixer 17 and the magenta fixer 18. Thethermal recording section includes a printing controller 36, and alsothe thermal head 16, an image memory group 31, a color corrector 32, aresistance change compensator 33, a line memory 34 and a comparator 35,which are sequentially controlled by the printing controller 36.

The image memory group 31 includes a yellow image memory 31 a, a magentaimage memory 31 b and a cyan image memory 31 c, which are controlled bya memory controller incorporated in the printing controller 36, forwriting and reading image data. An image to be printed is measured by ascanner (not shown) according to three-color separation photometry, andconverted into image data of 8 (eight) bits per color to be input to theimage memory group 31. Yellow image data is written to the yellow imagememory 31 a. Magenta image data is written to the magenta image memory31 b. Cyan image data is written to the cyan image memory 31 c. Inprinting operation, the three-color image data of one line to berecorded is read from the image memory group 31, and sent into the colorcorrector 32.

The color corrector 32 receives the three-color image data, effects thecolor correction, and sends the resistance change compensator 33 imagedata of each color to be recorded. Note that it is possible to writeRed, Green and Blue image data to the image memory group 31, and tooperate the color corrector 32 both for complementary color conversionand for the color correction, so as to obtain the yellow, magenta andcyan image data.

The resistance change compensator 33 includes an EEPROM (electricallyerasable programmable read-only memory) 33 a, a RAM (read-only memory)33 b and a ROM (random access memory) 33 c. EEPROM 33 a operates tostore resistance data Di (i=1, 2, . . . , n). ROM 33 c storesfundamental bias data of each color. The fundamental bias data iscompensated according to the resistance data Di. In the course ofassembly and adjustment of the thermal printer, the resistance data Diis determined by the resistance data measuring device 50 and written toEEPROM 33 a.

Inevitably there occurs variety in the resistance between the actualheating elements 20 ₁-20 _(n) of the heating element array 20, no matterprecisely their resistance is equally regulated during manufacturing.Changes or irregularities are created in a recorded image due to thevariety in the resistance, because heat energy from the heating elementshas conspicuous differences causing improper appearance in the image.Errors in the heat energy due to changes in the resistance include anerror in the bias heat energy occurring during the bias heating, and anerror in the image heat energy occurring during the image heating.

To eliminate bias heat energy error as an error in the bias heating, theresistance change compensator 33 utilizes the resistance data Di ofEEPROM 33 a to create one-line compensated bias data by compensating thefundamental bias data from ROM 33 c, and writes the compensated biasdata to RAM 33 b. Image heat energy error as an error, in the imageheating, depends on the highness of the image data. Once the one-lineimage data is written to RAM 33 b, the resistance change compensator 33calculates the image heat energy error in accordance with the image datastored in RAM 33 b and the resistance data Di. Again the compensatedbias data of RAM 33 b is compensated according to the image heat energyerror, to create one-line bias data. For the bias heating, theresistance change compensator 33 sends the one-line bias data to theline memory 34. For the image heating, the resistance change compensator33 reads the one-line image data from RAM 33 b, and sends it to the linememory 34.

To the line memory 34, the bias data or the image data for the one lineis written. The bias data or the image data for the one line is readfrom the line memory 34 sequentially pixel-by-pixel, and sent to thecomparator 35. The comparator 35 includes a counter for generatingcomparison data. In response to a count-up signal from the printingcontroller 36, the counter steps up the count by one. The number of thegrades of the gradation may be 256, so that the counter may generate thecomparison data of 1-255. The comparator 35 effects comparison betweenthe bias data or the image data for the one line and a series of thecomparison data, and successively pixel-after-pixel. If the bias data orthe image data is greater than the comparison data, then the comparator35 generates drive data of 1 (one). If the bias data or the image datais smaller than the comparison data, then the comparator 35 generatesdrive data of 0 (zero).

For the bias heating, the bias data of one line is compared for 255times with the comparison data of 1-255. The bias data of one pixel isconverted to drive data of, at most, 255 pulses. For the image heating,the image data of the one line is compared for 255 times. The image dataof one pixel is converted to drive data of at most 255 pulses. Thecomparator 35 outputs the one-line drive data in a serial form and sendsit to the thermal head 16.

A selector switch 38 is controlled by the system controller 30, and seton the side of a terminal 38 a for the thermal head, for directconnection of the thermal head 16 to a power source circuit 40, whichsupplies electric power to drive the heating elements 20 ₁-20 _(n). Theselector switch 38 is set on the side of a terminal 38 b for operationof resistance measurement. For determination of the resistance data Diof the heating elements 20 ₁-20 _(n), the terminal 38 b is operated forthe resistance data measuring device 50 to control the supply of thepower to the thermal head 16.

As shown in FIG. 2, the thermal head 16 includes a shift register 41, alatch array 42, an AND gate array 43, a heat control switch array 44 andthe heating element array 20. The shift register 41 fetches the one-lineserial drive data while shifting it successively upon the shift clockpulses, and converts it to the drive data of a parallel form, as anoutput toward the latch array 42.

The drive data having the parallel form from the shift register 41 islatched by the latch array 42 in synchronism with a latch signal, andsent to the gate array 43. A strobe signal for the bias heating is sentto the gate array 43 during the bias heating. A strobe signal for theimage heating is sent to the gate array 43 during the image heating. Thegate array 43 determines a logical product of the strobe signal and thedrive data from the latch array 42 at each bit, and sends the logicalproduct to the heat control switch array 44 via resistors 45 ₁-45 _(n).A drive pulse is generated at a width of the strobe signal if the bithas the drive data of 1, and is not generated if the bit has the drivedata of 0. The pulse width of the strobe signal is determined accordingto the characteristic curve of the recording sheet 13. The pulse widthof the strobe signal is set greater for the bias heating than for theimage heating. Also the pulse width of the strobe signal is setdifferent between the three colors. Note that the shift clock pulse, thelatch signal and the strobe signal are generated from the printingcontroller 36.

The heat control switch array 44 consists of heat control transistors 44₁-44 _(n), as switches respectively associated with the heating elements20 ₁-20 _(n). The heat control transistors 44 ₁-44 _(n) are turned onwhen a drive pulse is output from an associated one of the gate array43. An associated one of the heating elements is energized to apply heatfor printing by turning on of one of the heat control transistors 44₁-44 _(n).

A noise absorbing capacitor 46 is connected in parallel with the heatingelements 20 ₁-20 _(n). The capacitor 46 absorbs electrical noiseoccurring through a power source line between the thermal head 16 andthe power source circuit 40 disposed inside the thermal printer, andregulates the voltage applied to the heating element array 20. The heatenergy from the heating elements 20 ₁-20 _(n) would be changed accordingto changes of the applied voltage across the heating element array 20.The recording sheet could not be colored at desired density.

As shown in FIG. 2, the resistance data measuring device 50 includes aCPU 51 as a control circuit, a reference resistor 52, an additionaltransistor 53, a clock oscillator or clock generator (CG) 54, a counter55 as a timer, a window comparator 56 as a voltage detector, a chargingtransistor 58 and a resistor 64. The reference resistor 52 hasresistance Rs being known. The resistance data measuring device 50 alsoutilizes the capacitor 46 of thermal head 16 for the noise absorption,and measures the resistance data Di of the heating elements 20 ₁-20_(n). The reference resistor 52 is connected, in series, with theadditional transistor 53. The series connection between the referenceresistor 52 and the additional transistor 53 is connected, in parallel,with the capacitor 46 of the thermal head 16. The additional transistor53 is controlled by CPU 51 to be turned on/off in measuring theresistance data Di. The resistance Rs of the reference resistor 52 hashigh precision and high quality, and has small errors such as 1%. Thedetermination of the resistance data Di does not depend on theresistance Rs of the reference resistor 52, which can be set as desiredfor design.

The charging transistor 58 is connected between the power source circuit40 and the capacitor 46 as a charging switch for the capacitor 46. CPU51 keeps the charging transistor 58 turned off during the printing, andswitches the charging transistor 58 on and off during the measurement ofthe resistance.

The window comparator 56 detects a capacitor voltage Vc at which thecapacitor 46 is charged, and is constituted by a first comparator 56 a,a second comparator 56 b and an AND gate 56 c. One electrode of thecapacitor 46 is connected to an inverting input terminal of the firstcomparator 56 a and to a non-inverting input terminal of the secondcomparator 56 b. Potential dividing resistors 61, 62 and 63 areconnected in series for dividing potential of power supply voltage EH ofthe power source circuit 40. A non-inverting input terminal of the firstcomparator 56 a is connected to a line between the resistors 61 and 62.An inverting input terminal of the second comparator 56 b is connectedto a line between the resistors 62 and 63.

A predetermined high voltage V1, which is smaller than EH, is input to anon-inverting input terminal of the first comparator 56 a as a referencevoltage, which is obtained by potential division of the power supplyvoltage EH of the power source circuit 40. A predetermined low voltageV2, which is even smaller than V1, is input to an inverting inputterminal of the second comparator 56 b as a reference voltage, which isobtained by the potential division. Outputs of the comparators 56 a and56 b are sent to the AND gate 56 c, which outputs the signal CHG andsends it to CPU 51. The signal CHG, as an output of the windowcomparator 56, has a “High” level only when the capacitor voltage Vcacross the capacitor 46 is between the predetermined voltages V1 and V2.

While the charging transistor 58 is turned on, the capacitor 46 issupplied with electrical charge by the power source circuit 40 via theresistor 64, and charged. At the printing time, the selector switch 38is changed over to connect the power source circuit 40 directly to thepower source terminal of the thermal head 16. The power supply voltageEH of the power source circuit 40 is applied directly to the heatingelements 20 ₁-20 _(n). The heating elements 20 ₁-20 _(n) generate thegreater heat energy at one time because the current does not flow alongthe resistor 64. Time required for the printing is therefore preventedfrom being extended.

To measure the discharging time of the capacitor 46, the counter 55 isconnected to CPU 51. The clock generator 54 is connected to the counter55 for generating clock pulses of a constant frequency. A count Ck ofthe counter 55 is reset by CPU 51 to 0 (zero). CPU 51 controls thecounter 55 for starting and stopping of the counting operation.

In the measurement of the discharging time, CPU 51 operates to chargethe capacitor 46 according to the signal CHG until the capacitor voltageVc rises to the predetermined high voltage V1. Then the capacitor 46 isdischarged via the reference resistor 52 or a selected one of theheating elements 20 ₁-20 _(n). During the discharge, the counter 55counts the number of clock pulses generated while the capacitor voltageVc changes from the predetermined high voltage V1 to the predeterminedlow voltage V2. The count Ck obtained by measuring the referenceresistor 52 is discharging time Ts. The count Ck obtained by measuringthe heating elements 20 ₁-20 _(n) is discharging time Txi (i=1, 2, . . ., n). The count Ck in either case is written to a RAM 51 a. As will bedescribed later, CPU 51 includes a divider 51 c which effects arithmeticoperation to determine the resistance data Di of the heating elements 20₁-20 _(n) by use of the discharging times Ts and Txi stored in RAM 51 a,and sends the resistance data Di to the resistance change compensator33.

A ROM 51 b stores one-line drive data for the purpose of resistancemeasurement. To measure discharging time via each of the heatingelements 20 ₁-20 _(n), CPU 51 reads the one-line drive data from ROM 51b, to drive only a designated one of the heating elements. The drivedata is sent to the thermal head 16. Also CPU 51 generates the shiftclock pulse, the latch signal and the strobe signal. To measuredischarging time via the reference resistor 52, the additionaltransistor 53 is turned on.

The operation of the above construction is described now. The resistancedata Di is measured at the time of assembly and adjustment of the colorthermal printer. When the printer is initially powered, the systemcontroller 30 commands the resistance data measuring device 50 tomeasure the resistance data Di in response to checking selection of themeasurement at the selector switch 38.

When the measurement of the resistance data Di is commanded, CPU 51effects the sequences of FIGS. 6, 7 and 8 to measure the resistance dataDi of the heating elements 20 ₁-20 _(n). At first CPU 51 controlsoperations to measure the resistance of reference resistor 52, bymeasuring the discharging time Ts of the capacitor 46 via the referenceresistor 52. In FIG. 7, CPU 51 turns on the clock generator 54, whichsends the clock pulse to the counter 55 at the constant period.

Then CPU 51 resets the count Ck of the counter 55 as 0 (zero), and turnson the charging transistor 58. The thermal head 16 is supplied withcurrent by the power source circuit 40 via the resistor 64. Also thecapacitor 46 is charged. In FIG. 9, the capacitor voltage Vc across thecapacitor 46 gradually increases at a ratio of change according to theresistance of the resistor 64. When the capacitor voltage Vc exceeds thepredetermined low voltage V2, the signal CHG from the window comparator56 changes from the “Low” level to the “High” level. In the course offurther charging, the capacitor voltage Vc ultimately exceeds thepredetermined high voltage V1. In response, signal CHG becomes the“Low”. Immediately after the signal CHG becomes “Low”, CPU 51 turns offthe charging transistor 58, to stop charging the capacitor 46.

Note that the capacitor 46 can be pre-discharged in preparatory fashionby turning on the heat control transistors 44 ₁-44 _(n) of the heatcontrol switch array 44 before the charging transistor 58 is turned on.It is possible to avoid an erroneous operation in which the capacitor 46is charged with the capacitor voltage Vc accidentally exceeding thepredetermined low voltage V2.

It is possible to minimize the duration for charging the capacitor 46,by discontinuing the charge applied to the capacitor 46 when thecapacitor voltage Vc reaches the predetermined high voltage V1. Thecharging time depends on the capacitance of the capacitor 46, resistanceof the resistor 64, the power supply voltage EH and the predeterminedhigh voltage V1. According to the present embodiment, the charging timeis 2 msec. After the charging is stopped, CPU 51 turns on the additionaltransistor 53 connected to the reference resistor 52. In response,current flows between the reference resistor 52 and the capacitor 46,which is discharged via the reference resistor 52.

The capacitor voltage Vc across the capacitor 46 is gradually lowered bythe discharge. When the capacitor voltage Vc becomes slightly lower thanthe predetermined high voltage V1, the signal CHG becomes “High”. Inresponse, CPU 51 sends a command to the counter 55 to start counting.The counter 55 increments the count Ck by one each time one clock pulseis sent from the clock generator 54.

In the course of discharging the capacitor 46, the capacitor voltage Vcacross the capacitor 46 comes down to the predetermined low voltage V2.In response, the signal CHG becomes “Low”. The CPU 51 sends a stopcommand to the counter 55, to stop the counter 55 from counting.According to the present embodiment, the discharging time of the changeof the capacitor voltage Vc from the predetermined high voltage V1 tothe predetermined low voltage V2 is 10 msec.

After the counter 55 stops counting, CPU 51 turns off the additionaltransistor 53 to stop the discharge of the capacitor 46. Then CPU 51reads the count Ck from the counter 55, sets it for the discharging timeTs, and writes it to RAM 51 a. The discharging time Ts is a durationlapsed while the capacitor voltage Vc across the capacitor 46 changesfrom the predetermined high voltage V1 to the predetermined low voltageV2 during the discharge via the reference resistor 52.

After the discharging time Ts is stored, CPU 51 effects the sequence formeasuring the discharging time of the capacitor 46 via the heatingelements 20 ₁-20 _(n). At first CPU 51 measures the heating element 20₁. In FIG. 8, CPU 51 sends one-line drive data to the shift register 41in synchronism with the shift clock pulses. The one-line drive data usedpresently has 1 (one) only at a bit associated with the heat controltransistor 44 ₁, to turn on the heat control transistor 44 ₁ and turnoff the heat control transistors 44 ₂-44 _(n). Then CPU 51 sends thelatch signal to the latch array 42, which latches the one-line drivedata as set in the shift register 41.

After the latch signal is sent, CPU 51 resets the count Ck of thecounter 55 as 0 (zero), and then turns on the charging transistor 58.Again the capacitor 46 is charged. In a manner similar to the referenceresistor 52, the charging operation is stopped as soon as the capacitorvoltage Vc becomes the predetermined high voltage V1.

After the charging, CPU 51 sends the strobe signal to the gate array 43of the thermal head 16. The gate array 43 is provided by the latch array42 with the drive data for turning on the heat control transistor 44 ₁and turning off the heat control transistors 44 ₂-44 _(n). Upon supplyof the strobe signal from CPU 51, the gate array 43 sends a drive pulseto the heat control transistor 44 ₁. Only the heat control transistor 44₁ is turned on for the current to flow from the capacitor 46 to theheating element 20 ₁. The capacitor 46 is discharged via the heatingelement 20 ₁.

The capacitor voltage Vc across the capacitor 46 is gradually lowered bythe discharge. When the capacitor voltage Vc becomes slightly lower thanthe predetermined high voltage V1, the signal CHG becomes “High”. Likethe operation to measure the discharging time Ts at the referenceresistor 52, CPU 51 sends the start command to the counter 55, tomeasure the discharging time. When the capacitor voltage Vc comes downto the predetermined low voltage V2, the signal CHG becomes “Low”. CPU51 sends the stop command to the counter 55, to stop measuring thedischarging time.

After the counter 55 stops counting, CPU 51 stops sending the strobesignal, turns off the heat control transistor 44 ₁, and stops dischargevia the heating element 20 ₁. The count Ck is read, and written to RAM51 a as a discharging time Tx1 of the heating element 20 ₁. Thedischarging time Tx1 is time of the change of the capacitor voltage Vcfrom the predetermined high voltage V1 down to the predetermined lowvoltage V2 during the discharge of the capacitor 46 via the heatingelement 20 ₁.

CPU 51 starts the measurement of the heating element 20 ₂ next. CPU 51provides the shift register 41 with one-line drive data in synchronismwith the shift clock pulse. The one-line drive data is set to turn onthe heat control transistor 44 ₂ and turn off the heat controltransistors 44 ₁ and 44 ₃-44 _(n). CPU 51 sends the latch array 42 thelatch signal to latch the one-line drive data having been set in theshift register 41.

After the drive data is latched, the capacitor 46 is charged anddischarged via the heating element 20 ₂ in a similar manner to theheating element 20 ₁. A discharging time Tx2 is measured according tothe change in the signal CHG, and written to RAM 51 a. Repetitivelydischarging times Tx3, Tx4, . . . , Txn are measured with respect to theheating elements 20 ₃-20 _(n), and are written to RAM 51 a.

As described above, the discharging times Ts and Tx1-Txn associated withthe reference resistor 52 and the heating elements, 20 ₁-20 _(n) aremeasured. In charging the capacitor 46, the capacitor voltage Vc isdetected by the window comparator 56. The charging of the capacitor 46is stopped upon rise of the capacitor voltage Vc to the predeterminedhigh voltage V1. This is effective in shortening durations required formeasurement of the reference resistor 52 and the heating elements 20₁-20 _(n). It is possible for example to spend only 7 seconds inmeasuring the 512 heating elements. In the present embodiment, thedischarging time is measured according to the detection of the capacitorvoltage Vc at the window comparator 56. The capacitor voltage Vc can bemaintained at the predetermined high voltage V1 at the beginning of themeasurement. No error occurs in the measurement, as there are no changesin the power supply voltage or no irregularity in the capacitor voltage.

The capacitor 46 is prevented from being fully charged, as the capacitorvoltage Vc across the capacitor 46 is detected to limit the maximum atthe predetermined high voltage V1. It is possible to lower the voltageapplied to the heating elements 20 ₁-20 _(n) during the discharge, andto reduce the electrical stress to the heating elements 20 ₁-20 _(n) inthe measurement. The heating elements 20 ₁-20 _(n) are prevented fromdegradation. Note that the reference resistor 52 is measured beforemeasuring the heating elements 20 ₁-20 _(n), but may be measured aftermeasuring all the heating elements 20 ₁-20 _(n).

After the measurement of the discharging time Txi (i=1, 2, . . . , n) isfinished for all the heating elements 20 ₁-20 _(n), CPU 51 reads thedischarging times Ts and Txi from RAM 51 a. The divider 51 c as aresistance data determiner calculates the resistance data Di (i=1, 2, .. . , n) for each of the heating elements 20 ₁-20 _(n) according to theformula:

Di=Txi/Ts.

The resistance data Di, as determined is sent from CPU 51 to theresistance change compensator 33, and written to EEPROM 33 a. Theresistance data Di itself is not resistance of the heating elements 20₁-20 _(n), but represents a relative size of the resistance of theheating elements 20 ₁-20 _(n).

The definition above of the resistance data Di is described now indetail. The capacitor 46 is charged up to a certain voltage E that ishigher than the predetermined high voltage V1, and is discharged via thereference resistor 52 having the resistance Rs. Assuming the capacitor46 has capacitance C, a relationship between the capacitor voltage Vcacross the capacitor 46 and a discharging time t is expressed as:

Vc=E·exp(−t/Rs·C)  (1).

Assuming t1 represents the time taken for the capacitor voltage Vc todecreased to the predetermined high voltage V1 in the discharge of thecapacitor 46 via the reference resistor 52 and t2 represents the timetaken for the capacitor voltage Vc to decrease to the predetermined lowvoltage V2 in the discharge via the reference resistor 52, thepredetermined voltages V1 and V2 are expressed as follows:

V 1=E·exp(−t 1 /Rs·C)  (2),

and

V 2 =E·exp(−t 2/Rs·C)  (3).

The discharging time Ts is equal to a difference (t2−t1) of time, andthus is expressed as Formula (4). The resistance Rs is expressed asFormula (5).

Ts=t 2 −t 1 =C·Rs·ln(V 1/V 2)  (4).

Rs=Ts/[C·ln(V 1/V 2)]  (5).

The predetermined voltages V1 and V2 are supplied by division of thepotential of the power supply voltage EH. V1 and V2 can be defined byuse of coefficients E1 and E2:

 V 1=E1·EH; V 2=E2·EH.

Formula (5) is rewritten by use of a coefficient K1 depending onresistance of the resistors 61-63 and irrespective of the power supplyvoltage EH:

Rs=K1·Ts  (6),

where

K1=1/[C·ln(E1/E2)].

Similarly, resistance Rxi (i=1, 2, . . . , n) of the heating elements201-20n is expressed as:

Rxi=K1·Txi  (7),

where

K1=1/[C·ln(E1/E2)].

Formulae (6) and (7) result in the following formula, from which theresistance Rxi is obtained in accordance with the resistance Rs:.

Rxi=Txi/Ts·Rs  (8)

The resistance Rxi of the heating elements 20 ₁-20 _(n) does not dependon the power supply voltage EH at the measuring time. It is possible toprecisely to determine the resistance Rxi, even if the power supplyvoltage EH changes with time or changes each time of measuring one ofthe heating elements.

The heat energy errors depends on irregularity in the resistance Rxi ofthe heating elements. 20 ₁-20 _(n), but can be compensated for withoutuse of the resistance Rxi. In the color thermal printer, the ratiobetween the discharging time Ts via the reference resistor 52 and thedischarging time Txi via the heating elements 20 ₁-20 _(n) can bedetermined without determining the resistance Rxi, on the basis ofFormula (8). The resistance Rxi is proportional to Txi/Ts, so thatTxi/Ts is used as data Di. This is an advantage, since it allowsresistance Rxi to be determined indirectly, eliminating the time andeffort otherwise spent directly determining the resistance Rxi.

Now operation of the printing is described. At first, image data ofyellow, magenta and cyan of an image to be printed is written to theimage memory group 31. A manual operating panel (not shown) connected tothe system controller 30 is operated to command printing. The selectorswitch 38 is switched to the side of the thermal head. The heatingelements 20 ₁-20 _(n) are driven by the power source circuit 40 at thepower supply voltage EH.

The system controller 30 sends the printing command to the printingcontroller 36. The printing controller 36 sends the resistance changecompensator 33 a command for compensation of the three-color bias data.According to the resistance data Di (i=1, 2, . . . , n) of the heatingelements 20 ₁-20 _(n) written to EEPROM 33 a, the resistance changecompensator 33 compensates the fundamental bias data, which may be“240”, for yellow, to produce compensated bias-data of the heatingelements 20 ₁-20 _(n). Let Dm be average of the resistance data Di. Itis preferable, for example to obtain differences between the averageresistance data Dm and the resistance data Di, and to compensate thefundamental bias data according to the data differences, for producingthe compensated bias data.

If the resistance data Di is smaller than the average resistance dataDm, an amount of generated heat would be greater. Then the compensatedbias data is set “230”. Furthermore the average resistance data Dm ismultiplied by resistance of the reference resistor 52 to obtain averageresistance, to regulate a head voltage from the power source circuit 40.Note that it is possible to use maximum resistance data instead of theaverage resistance data Dm, and effect subtraction between “240” and avalue proportional to a difference between the resistance data Di andthe maximum resistance data.

Also for magenta and cyan, the resistance change compensator 33compensates the fundamental bias data according to the resistance dataDi (i=1, 2, . . . , n) of the heating elements 20 ₁-20 _(n), to producecompensated bias data of the heating elements 20 ₁-20 _(n). Thecompensated bias data of the three colors is written to RAM 33 b. Thusthe bias heat energy error, which is caused by irregularity inresistance of the heating elements 20 ₁-20 _(n) is compensated.

After the compensated bias data corresponding to the three colors iswritten to RAM 33 b, the printing controller 36 starts feeding therecording sheet 13, of which the front edge is retained on the peripheryof the platen drum 10 by the clamper 14. The platen drum 10 is rotatedto wind the recording sheet 13 about the platen drum 10.

The platen drum 10 makes an intermittent rotation step-by-step aspredetermined, until the front edge of a recording region of therecording sheet 13 comes to the thermal head 16. The thermal head 16 isswung down to press the heating element array 20 against the recordingsheet 13. Then an image starts being printed. At first, the three-colorimage data of the first line is read from the image memory group 31, andsent to the color corrector 32. The yellow image data of the first lineis subjected to the color correction in the color corrector 32 by takingthe three-color image data into consideration, and written to RAM 33 bof the resistance change compensator 33.

When the yellow image data of the first line is written to RAM 33 b, theresistance change compensator 33 adjusts the compensated bias data foryellow by use of the yellow image data and the resistance data Di, toremove an energy error in the image heating. Accordingly the bias dataof the first line of the yellow image is created. The image heat energyerror is greater in proportion to greatness of the image data, whichheightens the number of times of driving the heating elements in theimage heating. The bias heat energy, in accordance with the bias databeing created, is compensated by the resistance data Di and the yellowimage data. The bias heat energy is not necessarily equal to the biasheat energy Eby predetermined for the yellow.

The bias data for one line is sent to the line memory 34 successively,pixel after pixel, and written to it. After writing the one-line biasdata for all its pixels, the one-line bias data is read from the linememory 34 successively pixel-after-pixel, and sent to the comparator 35.The printing controller 36 causes a counter of the comparator 35 togenerate the comparison data of 1 (one).

The comparator 35 compares the bias data being input respectively withthe comparison data of 1 (one). If the bias data is equal to or greaterthan the comparison data of 1 (one), the comparator 35 generates thebias drive data of 1 (one). If not, the comparator 35 generates the biasdrive data of 0 (zero). The bias drive data of one line as obtained isoutput serially and sent to the thermal head 16. The serial drive datais converted by the shift register 41 into a parallel form of the biasdrive data.

The bias drive data of the parallel form is latched in the latch array42, which in turn sends the bias drive data of the parallel form to thegate array 43. After the latching at the latch array 42, the printingcontroller 36 sends the gate array 43 the bias strobe signal for yellow.The gate array 43 determines a logical product of the drive data of oneline and the bias strobe signal from the selector switch 38.

When a bias drive data is 1 (one), the bias drive pulse as wide as thebias strobe signal is sent to an associated one of the transistors inthe heat control switch array 44 through an output of the gate array 43associated with the bias drive data. The bias data is not compensated tothe value 0 (zero), so that any of the heat control transistors 44 ₁-44_(n) receives the first one of the bias drive pulses. The heat controltransistors 44 ₁-44 _(n) switch on the heating elements 20 ₁-20 _(n) ofthe heating element array 20 while the bias drive pulses are input tothem. The heating elements 20 ₁-20 _(n) are driven simultaneously andheated.

During the application of heat caused by the first one of the bias drivepulses, the printing controller 36 incrementally steps the counter ofthe comparator 35, which is caused to generate the comparison data of 2(two). Reading from the line memory 34 is effected for the second time.The one-line bias data is read from the line memory 34 successivelypixel-after-pixel, and sent to the comparator 35. In procedure asdescribed above, a second set of the bias drive pulses is produced forthe one line after heating caused by the first set of the bias drivepulses, to drive the heating elements 20 ₁-20 _(n) at the same time.

Similarly bias drive pulses are created by use of the comparison data of2-255, to drive the heating elements 20 ₁-20 _(n). Each of the heatingelements 20 ₁-20 _(n) is driven at times corresponding to the yellowbias data, at most at 255 times. If the yellow bias data is 248, then acorresponding one of the heating elements is driven at 248 times duringthe bias heating. The heat energy is applied to a position of one firstline of the recording sheet 13 at an amount of addition/subtraction ofthe bias heat energy error and the image heat energy error to/from thebias heat energy Eby.

After the bias heating, a first line of the yellow image data is readfrom RAM 33 b of the resistance change compensator 33, and written tothe line memory 34. Then the first line of the yellow image data is readfrom the line memory 34 pixel-after-pixel, and sent to the comparator35. In a manner similar to the bias drive data, the comparator 35compares the yellow image data being input respectively with thecomparison data of 1 (one). If the yellow image data is equal to orgreater than the comparison data of 1 (one), the comparator 35 generatesthe image drive data of 1 (one). If not, the comparator 35 generates theimage drive data of 0 (zero). The image drive data of one line asobtained is output serially and sent to the thermal head 16.

The image drive data of one line is converted to image drive pulses atthe thermal head 16 by use of the image strobe signal from the printingcontroller 36. If the image drive data is 0 (zero), then no image drivepulse is generated. The heating elements 20 ₁-20 _(n) of the heatingelement array 20 are selectively caused to heat by the image drivepulses of the one line by means of the heat control transistors 44 ₁-44_(n).

Similarly image drive pulses are created by use of the comparison dataof 2-255, to drive the heating elements 20 ₁-20 _(n) selectively. Eachof the heating elements 20 ₁-20 _(n) is driven at times corresponding tothe yellow image data, to generate heat. If the yellow printing isdesired at the highest density in a pixel, then a corresponding one ofthe heating elements 20 ₁-20 _(n) is driven at 255 times during theimage heating. If the yellow printing is desired at the lowest densityin a pixel, then a corresponding one of the heating elements 20 ₁-20_(n) is not driven.

This being so, the recording sheet 13 is subjected to the bias heatingand the image heating, and provided with the coloring heat energycorresponding to the yellow image data. According to the characteristiccurve in FIG. 4, the yellow coloring layer 26 is colored at the densityaccording to the image data, to print a dot within a pixel beingrectangular. If the equal yellow image data is printed with two heatingelements being different in resistance, the equal heat energy can begenerated from the two heating elements, to color the yellow-coloringlayer 26 at the equal density.

When the first line of the yellow image is recorded, the platen drum 10is rotated stepwise by one line. The three-color image data for a secondline is read from the image memory group 31. The yellow image data iswritten to RAM 33 b of the resistance change compensator 33. In a mannersimilar to the first line, the resistance change compensator 33compensates the compensated bias data for yellow written in RAM 33 b, inaccordance with the yellow image data of the second line and theresistance data Di. The bias data is obtained, and is used in the biasheating of a position on the recording sheet 13 of the second line.After the bias heating, the yellow image data of the second line fromRAM 33 b is written to the line memory 34. The yellow image data of thesecond line from RAM 33 b is read from the line memory 34, and used ineffecting the image heating of the second line. The second line isrecorded on the recording sheet 13.

Similarly third, fourth and fifth lines and so on of the yellow imageare recorded successively. After recording the yellow image, ultravioletrays of 420 nm are applied to the recording sheet 13 by the yellow fixer17. The yellow coloring layer 26 is optically fixed.

The platen drum 10 makes one rotation, again to move the recordingregion to the thermal head 16. A magenta image is recorded to the cyancoloring layer 24 line-by-line. Each pixel is recorded by combination ofthe bias heating and the image heating. In the bias heating, magentabias data is used after compensation by means of the magenta image dataand the resistance data Di. After recording the magenta image,ultraviolet rays of 365 nm are applied to the recording sheet 13 by themagenta fixer 18. The magenta coloring layer 25 is optically fixed.

The platen drum 10 makes another rotation to place the recording regionto the bottom of the thermal head 16. A cyan image is recorded to thecyan coloring layer 24 line-by line. In the bias heating, cyan bias datais used after compensation by means of the cyan image data and theresistance data Di. There is no operation of optical fixation of thecyan coloring layer 24. After finishing the cyan recording, therecording sheet 13 is exited to a tray of the printer.

A full color image is recorded on the recording sheet 13 in 256 steps ofthe gradation for each of the three colors. No irregularity in thedensity occurs in the full color image, even with the irregularityexisting in the resistance of the heating elements 20 ₁-20 _(n).

The resistance data Di is determined after finishing measurement of thedischarging time of all the heating elements. Also it is possible todetermine the resistance data Di each time by measuring the dischargingtime of one of the heating elements. Instead of the resistance data Di,resistance as obtained may be used, and compared with an idealizedresistance of an originally designed resistor to calculate a resistancedifference. The driving condition of the heating elements may becompensated according to the resistance difference. In the aboveembodiment, the reference resistor 52 is externally attached to thethermal head 16. Alternatively one of the heating elements can be usedas a reference resistor.

Another preferred embodiment is hereinafter described, in which theresistance data Di is obtained from charging time and discharging time.Elements similar to those of the above embodiment are designated withidentical reference numerals.

The resistance data measuring device 50 in FIG. 10 is similar to that ofFIG. 2, but has a reference resistor 70 instead of the resistor 64 anddoes not include the additional transistor 53 and the reference resistor52. A resistance Rq of the reference resistor 70 a high precisionresistor high quality similar to the reference resistor of the aboveembodiment, and resistor 70 producing small errors such as 1%. Thedetermination of the resistance data Di does not depend on theresistance Rq of the reference resistor 70, which can be set as desiredfor design.

The resistors 61-63 have such resistance that the predetermined voltagesV1 and V2 are determined as:

V 1=3/4·EH; V 2=1/4·EH,

for the purpose of simplifying an equation for obtaining the resistancedata Di.

CPU 51 measures a charging time and a discharging time of the capacitor46 using counter 55. When the charging time is measured, CPU 51 turns onthe charging transistor 58, charges the capacitor 46 via the referenceresistor 70, and causes the counter 55 to count the number of generatedclock pulses in accordance with signal CHG while the capacitor voltageVc charges from the predetermined low voltage V2 to the predeterminedhigh voltage V1. The count Ck of the counter 55 is written to RAM 51 aas a charging time Tq.

CPU 51 measures the discharging time Txi (i=1, 2, . . . , n) of thechange of the capacitor voltage Vc from the predetermined high voltageV1 down to the predetermined low voltage V2 during the discharge of thecapacitor 46 via each of the heating elements 20 ₁-20 _(n). Each timeafter the measurement of the charging time Tq and the discharging timeTxi (i=1, 2, . . . , n) for one of the heating elements 20 ₁-20 _(n),the divider 51 c calculates the resistance data Di (i=1, 2, . . . , n)for each of the heating elements 20 ₁-20 _(n) according to the formula:

Di=Txi/Tq.

Note that it is also possible to store the charging time Tq anddischarging time Txi obtained from the heating elements 20 ₁-20 _(n) inRAM 51 a, and to obbtain the resistance data Di after the dischargingtime Txi of all the heating elements 20 ₁-20 _(n).

In the resistance data measuring device 50 of FIG. 10, CPU 51 determinesthe resistance data Di of the heating elements 20 ₁-20 _(n) by followingthe routine of FIG. 11. CPU 51 starts the measurement for the first timeto measure the heating element 20 ₁. CPU 51 resets the counter 55 forthe count Ck to have 0 (zero), and turns on the charging transistor 58.The thermal head 16 is supplied with power by the power source circuit40 via the reference resistor 70, to charge the capacitor 46.

As FIG. 12, the capacitor voltage Vc across the capacitor 46 increasesat a ratio of change according to the resistance Rq of the referenceresistor 70. When the signal CHG comes over the predetermined lowvoltage V2 and becomes “High”, the counter 55 begins counting to measurethe charging time. When the capacitor voltage Vc exceeds thepredetermined high voltage V1 slightly during the charging and thesignal CHG becomes “Low”, CPU 51 stops the measurement of the chargingtime.

The charging time depends on the capacitor 46, the resistance Rq of thereference resistor 70, the power supply voltage EH and the predeterminedhigh voltage V1, and is 10 msec according to the present embodiment.After the stop of the charging, CPU 51 reads the count Ck from thecounter 55, and sets the count Ck as the charging time Tq, and writes itto RAM 51 a.

CPU 51 resets the count Ck as 0 (zero), and turns on only the heatcontrol transistor 44 ₁, to begin discharging the capacitor 46 via theheating element 20 ₁. When the signal CHG is slightly below thepredetermined high voltage V1 and becomes “High”, CPU 51 starts thecounter 55 to start measurement of the discharging time. During thedischarge, the signal CHG decreases to a level slightly below thepredetermined low voltage V2 and becomes “Low”. CPU 51 stops the counter55 to stop the measurement of the discharging time.

CPU 51 stops the discharge, reads the count Ck from the counter 55, setsthe count Ck as the discharging time Tx1, and writes discharging timeTx1 to RAM 51 a. Then CPU 51 reads the charging time Tq and dischargingtime Tx1 from RAM 51 a, and calculates resistance data D1 (=Tx1/Tq)associated with the heating element 20 ₁.

The resistance data D1 itself is not the resistance of the heatingelement 20 ₁, but corresponds closely to 20 ₁ the resistance of theheating element 20 ₁. The resistance data D1 is written to RAM 51 a.

CPU 51 effects measurement for the second time for the purpose ofobtaining resistance data D2 associated with the heating element 20 ₂.In a manner similar to the first time, the charging time Tq of thecapacitor 46 is measured via the reference resistor 70 in accordancewith the changes in the signal CHG. The discharging time Tx2 related tothe heating element 20 ₂ is measured. The resistance data D2 (=Tx2/Tq)is calculated from the charging time Tq and discharging time Tx2 asobtained, and is written to RAM 51 a. Similarly CPU 51 effectsmeasurement for the third, fourth, . . . , nth times, and obtainsresistance data D3, D4, . . . , Dn associated with the heating elements20 ₃-20 _(n).

The determination of the resistance data Di is described now in detail.The capacitor 46 is charged up to the power supply voltage EH via thereference resistor 70. Assuming the capacitor 46 has capacitance C, arelationship between the capacitor voltage Vc across the capacitor 46and a charging time t is expressed as:

Vc=EH·{1−exp(−t/Rq·C)}  (11).

Assuming t3 represents the time taken for the capacitor voltage Vc toreach the predetermined high voltage V1 in the charge of the capacitor46 via the reference resistor 70, and assuming t4 represents the timetaken for the capacitor voltage Vc to reach the predetermined lowvoltage V2 in the charge of the capacitor 46 via the reference resistor70 (t4<t3), the predetermined voltages V1 and V2 are expressed asfollows:

V 1=EH·{1−exp(−t 3/Rq·C)}  (12),

and

V 2=EH·{1−exp(−t 4/Rq·C)}  (13).

The charging time Tq is equal to a difference (t3−t4) of time, and thusis expressed as Formula (14). The resistance Rq is expressed as Formula(15). $\begin{matrix}\begin{matrix}{{Tq} = {{t3} - {t4}}} \\{= {{C \cdot {Rq} \cdot \ln}{\left\{ {\left( {{EH} - {V2}} \right)/\left( {{EH} - {V1}} \right)} \right\}.}}}\end{matrix} & (14)\end{matrix}$

 Rq=Tq/[C·ln{(EH−V 2)/(EH−V 1)}]  (15).

The resistance Rxi is defined in a manner similar to Formula (7) inmodification according to the capacitance C and the predeterminedvoltages V1 and V2, and is expressed by Formula (16):

Rxi=Txi/{C·ln(V 1/V 2)}  (16).

According to Formulae (15) and (16), a relationship between theresistance Rq of the reference resistor 70 and the resistance Rxi isexpressed as:

Rxi=Txi/Tq·Rq/K2  (17),

where

K2={ln(V 1/V 2)}/[ln {(EH−V 2)/(EH−V 1)}]}.

In relation to Formula (17), a coefficient K2 depends on resistance ofthe resistors 61-63 and irrespective of the power supply voltage EH. Thecoefficients E1 and E2 are used again to define V1 and V2:

V 1=E1·EH; V 2=E2·EH.

Therefore,

K2={ln(E1/E2)}/[ln{(1−E2)/(1−E1)}]}

The resistance Rxi of the heating elements 20 ₁-20 _(n) does not dependon the power supply voltage EH at the measuring time. It is thereforepossible precisely to determine the resistance Rxi.

If E1 and E2 satisfy the relationship:

 E1² −E2² −E1+E2=0,

namely

E1/E2=(1−E2)/(1−E1) where E1>E2,

then the coefficient K2 is 1 (one), to simplify arithmetic operation ofobtaining the resistance Rxi. In the present embodiment, the resistanceof the resistors 61-63 is determined to satisfy

E1=3/4; E2=1/4,

so that K2=1. The resistance Rxi of the heating elements 20 ₁-20 _(n) isobtained from the following formula:

Rxi=Txi/Tq·Rq  (18.)

It is unnecessary to calculate the resistance Rxi of the heatingelements 20 ₁-20 _(n). The resistance Rxi is proportional to Txi/Tq ofwhich the discharging time Txi is obtained from the heating elements 20₁-20 _(n), and the charging time Tq is obtained from the referenceresistor 70. Hence the Txi/Tq is used as the resistance data Di.

It is therefore possible to shorten the durations required formeasurement of the heating elements 20 ₁-20 _(n). Only 10 seconds isspent in measuring the 512 heating elements. There is only a slightinterval between the discharging time and the charging time beingmeasured, so that the precision in the measurement can be increased.

In the above embodiments, the number of the bias drive pulses arechanged to compensate for the heat energy errors including the bias heatenergy error and the image heat energy error. It is also possible tochange the width of the bias drive pulses to compensate for the heatenergy errors. Furthermore, the number or width of the bias drive pulsescan be changed for compensating for the bias heat energy error. Thenumber or width of the image drive pulses can be changed forcompensating for the image heat energy error.

The present invention could be applied to a separate type of resistancedata measuring device which would be a part of the above-describedthermal printer, and which would be detached from it. This resistancedata measuring device may be used as a tool or instrument for inspectingthe thermal head.

In the above embodiments, the resistance data is measured in the factoryfor adjustment after manufacture and assembly. The present invention isalso applicable to an automatic measuring operation of the resistancedata. The automatic operation can be triggered in the printer uponpowering the printer by a user's manual operation, or upon lapse of aperiod predetermined suitably.

In the above embodiments, the capacitor is charged up to thepredetermined high voltage V1 at the highest. It is also possible tocharge the capacitor to a voltage slightly higher than the predeterminedhigh voltage V1.

The above embodiments are applied to a color thermal printer of a directrecording type. The present invention is also applicable to amonochromatic thermal printer, or a color thermal printer of a thermaltransfer type. The above embodiments are directed to a line printer inwhich the thermal head is moved one-dimensionally relative to therecording sheet. The present invention is applicable to a serial printerin which the thermal head is moved two-dimensionally relative to therecording sheet. The present invention is also applicable to athree-head/one-pass type of color thermal printer including threethermal heads for the yellow, magenta and cyan.

In the above embodiments, the resistance data of all the heatingelements are determined. The present invention is applicable todetermining resistance data of only at least one of the heatingelements. For this operation, the discharging time measured via thereference resistor 52 and discharging time measured via the at least oneheating element are considered, or the charging time measured via thereference resistor 70 and discharging time measured via the at least oneheating element are considered.

Although the present invention has been fully described by way of thepreferred embodiments thereof with reference to the accompanyingdrawings, various changes and modifications will be apparent to thosehaving skill in this field. Therefore, unless otherwise these changesand modifications depart from the scope of the present invention, theyshould be construed as included therein.

What is claimed is:
 1. A resistance data measure device for a thermalhead, said thermal head having plural heating elements arranged along aline and connected in parallel with one another, a respective one ofsaid heating elements being connected to one of plural heat controlswitches, said heat control switches being selectively turned on/off fordriving said heating elements individually by applying electrical energyto generate heat, said resistance data measure device comprising: acapacitor connected in parallel with said plural heating elements; acharging switch for charging said capacitor; a voltage detector fordetecting a capacitor voltage across said capacitor; a referenceresistor connected in parallel with said plural heating elements andsaid capacitor; an additional switch connected to said referenceresistor; a control circuit for controlling said heat control switches,said charging switch and said additional switch; said control circuitinitially turning on said charging switch to charge said capacitor;turning off said charging switch to stop charging said capacitor upon anincrease of said capacitor voltage to a predetermined high voltage; andthereafter turning on one selected from a group including saidadditional switch and said plural heat control switches, to dischargesaid capacitor via said reference resistor or one of said heatingelements in association with said selected one being turned on; a timerfor measuring discharging time elapsed in a decrease of said capacitorvoltage from said predetermined high voltage to a predetermined lowvoltage while said capacitor is discharged, in association respectivelywith said reference resistor and said heating elements; and a resistancedata determiner for determining resistance data of said heating elementsin accordance with said discharging time respectively of said heatingelements with reference to said discharging time of said referenceresistor.
 2. A resistance data measure device as defined in claim 1,wherein said resistance data is a ratio of said discharging time of eachof said heating elements to said discharging time of said referenceresistor, and represents a relative greatness of resistance.
 3. Aresistance data measure device as defined in claim 2, wherein saidvoltage detector comprises a window comparator supplied with inputs ofsaid capacitor voltage, a first reference voltage set equal to saidpredetermined high voltage, and a second reference voltage set equal tosaid predetermined low voltage, for comparing said capacitor voltagewith said first reference voltage and with said second referencevoltage, said window comparator generating a detection signal when saidcapacitor voltage is between said first and second reference voltages;said timer is started in response to a start of generation of saiddetection signal from said window comparator, and stopped in response toan end of said generation of said detection signal from said windowcomparator.
 4. A resistance data measure device as defined in claim 3,said resistance data measure device being incorporated in a thermalprinter in which said heating elements are respectively driven by adrive signal based on bias data and image data, to effect thermalrecording to recording material; said thermal printer further includinga compensator for compensating said drive signal associated with saidheating elements, in accordance with said resistance data determined bysaid resistance data determiner.
 5. A resistance data measure device fora thermal head, said thermal head having plural heating elementsarranged along a line and connected in parallel with one another, arespective one of said heating elements being connected to one of pluralheat control switches, said heat control switches being selectivelyturned on/off for driving said heating elements individually by applyingelectrical energy to generate heat, said resistance data measure devicecomprising: a capacitor connected in parallel with said plural heatingelements; a charging switch for charging said capacitor; a voltagedetector for detecting a capacitor voltage across said capacitor; areference resistor connected to said charging switch in series; acontrol circuit for controlling said heat control switches and saidcharging switch; said control circuit initially turning on said chargingswitch to charge said capacitor via said reference resistor; turning offsaid charging switch to stop charging said capacitor; and thereafterturning on one selected from said plural heat control switches, todischarge said capacitor via one of said heating elements in associationwith said selected one being turned on; a timer for measuring chargingtime elapsed in an increase of said capacitor voltage from apredetermined low voltage to a predetermined high voltage while saidcapacitor is charged, and for measuring discharging time elapsed in adecrease of said capacitor voltage from said predetermined high voltageto said predetermined low voltage while said capacitor is discharged, inassociation respectively with said heating elements; and a resistancedata determiner for determining resistance data of said heating elementsin accordance with said discharging time with reference to said chargingtime.
 6. A resistance data measure device as defined in claim 5, whereinsaid control circuit turns off said charging switch upon an increase ofsaid capacitor voltage to said predetermined high voltage while saidcharging switch is turned on.
 7. A resistance data measure device asdefined in claim 6, wherein said resistance data is a ratio of saiddischarging time of each of said heating elements to said charging timeof said reference resistor, and represents a relative greatness ofresistance.
 8. A resistance data measure device as defined in claim 7,wherein said voltage detector comprises a window comparator suppliedwith inputs of said capacitor voltage, a first reference voltage setequal to said predetermined high voltage, and a second reference voltageset equal to said predetermined low voltage, for comparing saidcapacitor voltage with said first reference voltage and with said secondreference voltage, said window comparator generating a detection signalwhen said capacitor voltage is between said first and second referencevoltages; said timer is started in response to a start of generation ofsaid detection signal from said window comparator, and stopped inresponse to an end of said generation of said detection signal from saidwindow comparator.
 9. A resistance data measure device as defined inclaim 8, said resistance data measure device being incorporated in athermal printer in which said heating elements are respectively drivenby a drive signal based on bias data and image data, to effect thermalrecording to recording material; said thermal printer further includinga compensator for compensating said drive signal associated with saidheating elements, in accordance with said resistance data determined bysaid resistance data determiner.
 10. A thermal printer, including athermal head having plural heating elements arranged along a line andconnected in parallel with one another, a respective one of said heatingelements being connected to one of plural heat control switches, saidheat control switches being selectively turned on/off, for driving saidheating elements individually by applying a drive signal based on biasdata and image data, to effect thermal recording to recording material,said thermal printer comprising: a capacitor connected in parallel withsaid plural heating elements; a charging switch for charging saidcapacitor; a voltage detector for detecting a capacitor voltage acrosssaid capacitor; a reference resistor connected in parallel with saidplural heating elements and said capacitor; an additional switchconnected to said reference resistor; a control circuit for controllingsaid heat control switches, said charging switch and said additionalswitch; said control circuit initially turning on said charging switchto charge said capacitor; turning off said charging switch to stopcharging said capacitor upon an increase of said capacitor voltage to apredetermined high voltage; and thereafter turning on one selected froma group including said additional switch and said plural heat controlswitches, to discharge said capacitor via said reference resistor or oneof said heating elements in association with said selected one beingturned on; a timer for measuring discharging time elapsed in a decreaseof said capacitor voltage from said predetermined high voltage to apredetermined low voltage while said capacitor is discharged, inassociation respectively with said reference resistor and said heatingelements; a resistance data determiner for determining resistance dataof said heating elements in accordance with said discharging timerespectively of said heating elements with reference to said dischargingtime of said reference resistor; and a compensator for compensating saiddrive signal associated with said heating elements, in accordance withsaid resistance data determined by said resistance data determiner. 11.A thermal printer, including a thermal head having plural heatingelements arranged along a line and connected in parallel with oneanother, a respective one of said heating elements being connected toone of plural heat control switches, said heat control switches beingselectively turned on/off, for driving said heating elementsindividually by applying a drive signal based on bias data and imagedata, to effect thermal recording to recording material, said thermalprinter comprising: a capacitor connected in parallel with said pluralheating elements; a charging switch for charging said capacitor; avoltage detector for detecting a capacitor voltage across saidcapacitor; a reference resistor connected to said charging switch inseries; a control circuit for controlling said heat control switches andsaid charging switch; said control circuit initially turning on saidcharging switch to charge said capacitor via said reference resistor;turning off said charging switch to stop charging said capacitor; andthereafter turning on one selected from said plural heat controlswitches, to discharge said capacitor via one of said heating elementsin association with said selected one being turned on; a timer formeasuring charging time elapsed in an increase of said capacitor voltagefrom a predetermined low voltage to a predetermined high voltage whilesaid capacitor is charged, and for measuring discharging time elapsed ina decrease of said capacitor voltage from said predetermined highvoltage to said predetermined low voltage while said capacitor isdischarged, in association respectively with said heating elements; aresistance data determiner for determining resistance data of saidheating elements in accordance with said discharging time with referenceto said charging time; and a compensator for compensating said drivesignal associated with said heating elements, in accordance with saidresistance data determined by said resistance data determiner.